1. Field
The invention relates to a corrosion resistant substrate, particularly a substrate with a Cr(VI) free corrosion resistant coating and a method for producing the same.
2. Description of Related Art
Metal sheets and metal parts, for example made of steel and aluminum, are often provided with a coating which protects the sheet or part against corrosion by corrosive media and oxygen. Furthermore, with the support of this coating the adhesion of paints applied thereto may be improved, further improving the corrosion resistance of this part. The corrosion protection incorporating the coating is tested according to specified test conditions, such as tests applying atomized salt spray according to DIN 50 021 SS or outdoor weathering.
Some corrosion protection coatings comprise Cr(VI) containing compositions. However, because of the toxicity of Cr(VI), coatings containing Cr(VI) are no longer desirable. Consequently, Cr(VI) free substitute products, as for example described in U.S. Pat. No. 6,375,726 have been developed throughout the last years.
Some of the Cr(VI) free coating substitutes with acceptable corrosion protection properties already exist for standard corrosion protection conditions. However, the corrosion resistance of these Cr(VI) free coatings is inadequate for some substrate materials and in highly corrosive environments.
It is known from further testing that the corrosion protection of the currently available Cr(VI) free coatings is inadequate in highly corrosive acid containing atmospheres. For example, an acid containing atmosphere develops in exhaust gas systems of vehicles, particularly in exhaust gas systems with exhaust gas recirculation and waste gas systems. These applications comprise the further requirement that the coating also has to be corrosion resistant at higher temperatures, for example 120° C. or up to 250° C. However, the already developed Cr(VI) free coatings show signs of corrosion after a short amount of time under these conditions.